The invention relates to a servo-assisted steering system with hydraulic servo-assistance for motor vehicles. Here, a steering shaft, which is provided with a steering wheel, is connected via a torsion spring with an input member of a steering gear. An output member of the steering gear is connected with the wheels of the vehicle which are to be steered. In case of a relative rotation of the steering shaft in respect to the input member, two work chambers of a servo motor of a force amplification device can be pressurized by a servo pump via a steering valve with a pressure medium. At least one sensor for detecting a rotation angle and/or a torque is arranged on the steering shaft. A sensor for detecting a steering angle is arranged on the output member of the steering gear. This sensor can also be integrated into the rotation angle sensing mechanism of the steering shaft. A returning device is used for the hydraulic assistance of the returning of the steering gear and the wheels to be steered after the steering wheel is released following a steering process.
Such a servo-assisted steering system is known from EP 0 440 63 8 B1. In this case an electric motor used to affect the reaction force, which can be noted on the steering wheel, as a function of various parameters, is arranged in addition to a complete servo-assisted steering system with hydraulic servo-assistance. Moreover, the electric motor is used to center the steering valve exactly in its center. Finally, it is possible to achieve a hydraulic steering axle return in that, following the release of the steering wheel, the steering valve is displaced past its neutral position into a steering position, which corresponds to a reversal of the steering direction. A steering system is known from U.S. Pat. No. 4,745,985, which contains several features of the invention. With this steering system the amount and the direction of the hydraulic servo-assistance, as well as the return of the wheels to be steered, are controlled by means of a common electronically-controlled electromagnetic valve. Steering errors could appear in case of the possible appearance of a malfunction in the electronic control device, which could lead to endangerment.